A wheel with 10 metallic spokes each 0.3 m long is rotated with a speed of 120 rev/min in a plane normal to the horizontal component of earth’s magnetic field $H_E$ at a place. If $H_E$ = 0.4 G at the place, what is the induced emf between the axle and the rim of the wheel? Note that 1 G = $10^{–4}$ T.

$\begin{array}{1 1} 2.26 \times 10^{-5}V \\ 3.14 \times 10^{-5}V \\ 1.57 \times 10^{-5}V \\ 4.71 \times 10^{-5}V \end{array}$

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• The number of spokes is immaterial because the emf’s across the spokes are in parallel.
Induced emf $\varepsilon = \large\frac{1}{2}$$\omega B R^2 \rightarrow$$\varepsilon = \large\frac{1}{2}$$4\;\pi \times 0.4 \times 10^{-4} \times 0.3^2 = 2.26 \times 10^{-5} V$
answered Mar 10, 2014
edited Jul 14, 2014